Definition from the The Star Trek Encyclopedia: A Reference Guide to the Future 2nd edition...

Chief Medical Officer. Aboard a Federation starship, the officer in charged with the responsibility for the health and well-being of the ship's crew. Under certain circumstances, the CMO is authorized to certify a ship's Captain as unfit for command and relieve them on that basis.
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The chief medical officer is an officer who is directly in charge of a starship's sickbay or infirmary on a starbase. This officer is in charge of all medical personnel at the facility and oversees the health of the crew.
The CMO also has the power to relieve their commanding officer regardless of the CMO's rank if they judge the CO of been unfit for duty. (TOS: "The Doomsday Machine"; TNG: "Lonely Among Us"; VOY: "Year of Hell, Part II")
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